Age-dependent structural optimization of biogenic hydroxyapatite: implications for the design of nanocrystalline bone grafts and implant coatings

This work investigates the age-dependent structural and compositional evolution of biogenic hydroxyapatite (HAp) extracted from the femurs of male Wistar rats aged 3 to 40 weeks, aiming to support the selection and design of materials from biogenic sources for bone regeneration applications. The HAps samples were characterized by TGA/DSC, FTIR, Rietveld-refined XRD, ICP-OES, and SEM, reveling significant age-related changes in crystallinity, porosity, lattice parameters, and elemental composition. The results showed that Ca and P content and crystallite size increased with age, with a hydroxylation process in the neonatal stage reaching optimal values around 25 weeks—indicating that HAp at this age has the best structural stability and chemical integrity. In contrast, older bones (≥40 weeks) showed lower mineral quality and higher porosity. These findings demonstrate donor age strongly influence the physicochemical properties of HAp, directly affecting its suitability for biomedical applications such as bone grafts and implant coatings. The study also highlights the importance of adapting the calcination process of the materials to the age of the donor in order to preserve the nanostructures or promote crystallite growth. Based on these results, this work provides a foundation for designing bioceramics based on HAp, improving reproducibility, bioactivity, and loading capacity in regenerative medicine.